As generally known in the art, a joint functions to transmit rotational power (torque) between two rotation shafts which meet each other at an angle. In the case of a propeller shaft having a small power transmission angle, a hook joint, a flexible joint, etc. are used, and in the case of the driving shaft of a front wheel drive vehicle having a large power transmission angle, a constant velocity joint is used.
Because the constant velocity joint can reliably transmit power at a constant velocity even when an angle between a driving shaft and a driven shaft is large, the constant velocity joint is mainly used for the axle shaft of an independent suspension type front wheel drive vehicle. When viewed from a shaft, a tripod type constant velocity joint is provided to one end of the shaft which faces an engine, and a Birfield type constant velocity joint is provided to the other end of the shaft which faces a tire.
FIG. 1 is a cross-sectional view illustrating conventional constant velocity joints, and FIG. 2 is a cross-sectional view taken along the line A-A of FIG. 1. Referring to FIGS. 1 and 2, the conventional constant velocity joints comprise a tripod type constant velocity joint which is provided to the right end of a shaft 1 (which faces an engine) and a Birfield type constant velocity joint provided to the left end of the shaft 1 (which faces a tire).
The tripod type constant velocity joint installed on the right end of the shaft 1 (which faces the engine) comprises a housing 2 which transmits rotational power of the engine (not shown) and is defined with track grooves on the inner surface thereof, the shaft 1 which receives the rotational power from the housing 2 and rotates, a spider 3 which is disposed in the housing 2, is coupled to one end of the shaft 1 to connect the housing 2 and the shaft 1 with each other and is formed with three trunnions to be respectively inserted into the track grooves of the housing 2, needle rollers 6 which are arranged on the circumferential outer surface of each trunnion of the spider 3, inner rollers 5 each of which is arranged around the needle rollers 6 for each trunnion of the spider 3, outer rollers 4 each of which is installed on the circumferential outer surface of each inner roller 5 to reduce friction between the housing 2 and the shaft 1, a retainer ring 8 which is installed on the upper ends of the needle rollers 6 and of each inner roller 5, a boot 10 having one end which is connected to the housing 2 and the other end which is connected to the shaft 1, and clamping bands 11 and 12 which clamp both ends of the boot 10.
The Birfield type constant velocity joint installed on the left end of the shaft 1 (which faces the tire) comprises an inner race 15 which is installed on the left end of the shaft 1 to receive the rotational power from the tripod type constant velocity joint and to then rotate, an outer race 13 which is installed around the inner race 15, balls 16 for transmitting the rotational power of the inner race 15 to the outer race 13, a cage 14 for supporting the balls 16, a sensor ring 17 which is installed around the outer race 13, a boot 18 having one end which is connected to the shaft 1 and the other end which is connected to the outer race 13, and clamping bands 19 and 20 which clamp both ends of the boot 18.
Hereafter, the operation of the conventional constant velocity joints constructed as mentioned above will be described.
As the rotational power outputted from the engine is transmitted to the housing 2 through a transmission, the housing 2 is rotated. The rotational power of the housing 2 is transmitted to the spider 3 through the outer rollers 4, the inner rollers 5 and the needle rollers 6, and then the shaft 1 to which the spider 3 is coupled is rotated. The rotational power of the shaft 1 is transmitted to the outer race 13 through the inner race 15 and the balls 16, and then the wheel (not shown) connected to the outer race 13 is rotated.
In the tripod type constant velocity joint which is provided to the right end of the shaft 1 (which faces the engine), as the outer rollers 4 slide in the track grooves of the housing 2, the rotation angle of the shaft 1 which is operationally associated with the outer rollers 4 is changed to follow the movement of a vehicle. In the Birfield type constant velocity joint which is provided to the left end of the shaft 1 (which faces the tire), the rotation angle of the outer race 13 is changed due to the presence of the balls 16 to follow the movement of the vehicle.
The boot 10 of the tripod type constant velocity joint and the boot 18 of the Birfield type constant velocity joint respectively function to enclose the tripod type constant velocity joint and the Birfield type constant velocity joint, so that the tripod type constant velocity joint and the Birfield type constant velocity joint are prevented from being contaminated by foreign substances.
FIG. 3 is a cross-sectional view illustrating another conventional tripod type constant velocity joint which has a different construction from the tripod type constant velocity joint shown in FIG. 1 Referring to FIG. 3, another conventional tripod type constant velocity joint comprises a housing 2′ which is defined with three track grooves each having an optionally contoured guide surface, a spider 3′ which is projectedly formed with three spherical trunnions 3a to be respectively inserted into the track grooves of the housing 2′, inner rollers 5′ each of which is installed to surround each spherical trunnion 3a, with the surface thereof to be brought into contact with the spherical trunnion 3a having a concave contour, a plurality of needle rollers 6′ which are arranged around each inner roller 5′, outer rollers 4′ each of which is rotated by the medium of the needle rollers 6′, and a retainer ring 8′ which is installed to prevent the needle rollers 6′ from being released.
The operation of the conventional tripod type constant velocity joint constructed as just mentioned above will be described below.
As power is transmitted to the housing 2′ and the housing 2′ is rotated, the power is transmitted to the trunnions 3a through the outer rollers 4′, the needle rollers 6′ and the inner rollers 5′ to rotate the spider 3′. At this time, the combination of the inner roller 5′ and the outer roller 4′, which are operationally connected with each other through the needle rollers 6′ and are rotated relative to each other, is guided along the guide surface of the housing 2′ in the axial direction of the track groove of the housing 2′. Self-aligning movement (center-adjusting oscillation) occurs between the concave contour of the inner roller 5′ and the spherical trunnion 3a. 
However, in the case that the self-aligning movement occurs to absorb and correct the tilt of the spider 3′ via the spherical trunnions 3a, as can be readily seen from FIGS. 4 and 5, since the contact area between the concave contour of the inner roller 5′ and the spherical trunnion 3a, which are moved relative to each other, is substantial, frictional force generated therebetween increases. Also, because the relative movement decreases at the point 3c where the axis 3d of the self-aligning movement and the outer surface of the spherical trunnion 3a meet with each other, if the corresponding component elements are continuously rotated while receiving a load, insufficient lubrication can result and the rotational durability of the constant velocity joint can be degraded.
FIG. 6 is a transverse cross-sectional view illustrating still another conventional tripod type constant velocity joint, and FIG. 7 is a cross-sectional view taken along the line I-I of FIG. 6 Referring to FIGS. 6 and 7, still another conventional tripod type constant velocity joint comprises a housing 2″ which is defined with three track grooves each having an optionally contoured guide surface, a spider 3″ which is projectedly formed with three elliptical trunnions 3e to be respectively inserted into the track grooves of the housing 2″, inner rollers 5″ each of which is installed to surround each elliptical trunnion 3e, with the surface thereof to be brought into contact with the elliptical trunnion 3e having a convex contour, a plurality of needle rollers 6″ which are arranged around each inner roller 5″, outer rollers 4″ each of which is rotated by the medium of the needle rollers 6″, and a retainer ring 8″ which is installed to prevent the needle rollers 6″ and the inner rollers 5″ from being released.
The operation of the conventional tripod type constant velocity joint constructed as just mentioned above will be described below.
As power is transmitted to the housing 2″ and the housing 2″ is rotated, the power is transmitted to the elliptical trunnions 3e through the outer rollers 4″, the needle rollers 6″ and the inner rollers 5″ to rotate the spider 3″. At this time, the combination of the inner roller 5″ and the outer roller 4″, which are operationally connected with each other through the needle rollers 6″ and are rotated relative to each other, is guided along the guide surface of the housing 2″ in the axial direction of the track groove of the housing 2″. Self-aligning movement (center-adjusting oscillation) occurs between the convex contour of the inner roller 5″ and the elliptical trunnion 3e. 
However, when the elliptical trunnion 3e and the convex contour of the inner roller 5″ are operationally connected with each other, as can be readily seen from FIGS. 7 and 8, since the elliptical trunnion 3e and the convex contour of the inner roller 5″ are brought into contact with each other on one point 3f to transmit power, a problem is caused in that surface pressure increases and the durability of the constant velocity joint is deteriorated. Also, because power is transmitted through point contact, the outer roller 4″ cannot be held parallel in the track groove of the housing 2″, whereby power transmission stability cannot be ensured and the effect of suppressing the creation of additional force components by driving force cannot be guaranteed.